Optically compensated electro-optical light modulation element with optically isotropic phase

ABSTRACT

The invention relates to a compensated electro-optical light modulation element comprising a mesogenic medium that is operated in an optically isotropic phase of the mesogenic medium and comprising at least one compensation element, and to displays containing such a light modulation element.

FIELD OF THE INVENTION

The invention relates to a compensated electro-optical light modulationelement comprising a mesogenic medium that is operated in an opticallyisotropic phase of the mesogenic medium and comprising at least onecompensation element, and to displays containing such a light modulationelement.

BACKGROUND AND PRIOR ART

The liquid-crystal display (LCD) devices typically used in prior art arefor example TN (twisted nematic) LCDs, for example in accordance withSchadt, M. and Helfrich, W. Appl. Phys. Lett. 18, pp. 127 ff (1974) andin particular in their special form with low optical retardation d·Δn inthe range from 150 nm to 600 nm in accordance with DE 30 22 818, STN(super twisted nematic) LCDs, such as, for example, in accordance withGB 2.123.163, Waters, C. M., Brimmel, V, and Raynes, E. Proc. 3^(rd)Int. Display Research Conference, Kobe 1983, pp. 396 ff and Proc. SID25/4, pp. 261 ff, 1984, Scheffer, T. J. and Nehring, J. Appl. Phys.Lett. 45, pp. 1021 ff, 1984 and J. Appl. Phys. 58, pp. 3022 ff, 1985, DE34 31 871, DE 36 08 911 and EP 0 260 450, IPS (in-plane switching) LCDs,as described, for example, in DE 40 00 451 and EP 0 588 568, and VA orVAN (vertically aligned nematic) LCDs, as described, for example, inTanaka, Y. et al. Taniguchi, Y., Sasaki, T., Takeda, A., Koibe, Y., andOkamoto, K. SID 99 Digest pp. 206 ff (1999), Koma, N., Noritake, K.,Kawabe, M., and Yoneda, K., International Display Workshop (IDW) '97 pp.789 ff (1997) and Kim, K. H., Lee, K., Park, S. B., Song, J. K., Kim,S., and Suk, J. H., Asia Display 98, pp. 383 ff, (1998).

In LCD devices which were known hitherto and are for the most partalready commercially available, the optical appearance is inadequate, atleast for demanding applications. In particular the contrast, especiallyin the case of coloured displays, the brightness, the colour saturationand the viewing-angle dependence of these parameters are in clear needof improvement and have to be improved if the display devices are tocompete with the performance features of the widespread CRTs (cathoderay tubes). Further disadvantages of the LCD devices of prior art areoften their poor spatial resolution and inadequate response times, inparticular in the case of STN, but also in the case of TN or IPS and VALCDs, in the case of the latter especially if they are to be used forthe reproduction of video, such as, for example, in multimediaapplications on computer display screens or in the case of televisionsets. Particularly for this purpose, but also even for the display ofrapid cursor movements, short response times are desired.

Recently a new type of LCDs and LC devices has been reported in priorart. These devices are disclosed for example in WO 02/93244 A1, DE10217273 A1, DE 102 41 301 or DE 103 139 79, and are hereinafter alsoreferred to as ISP (isotropic switching panel) mode or device. Theycomprise an electro-optical light modulating element containing anelectrically switchable mesogenic or liquid crystal (LC) medium that isoperated in an optically isotropic phase, like for example in theisotropic phase (i.e. at a temperature above the clearing point of theLC medium) or, more preferably, in a blue phase. The mesogenic or LCmedium in the optically isotropic phase becomes birefringent when anelectric field is applied. Interdigitated electrodes on one side of thelight modulation element create an in-plane electric field parallel tothe plane of the element, which aligns the mesogenic or LC molecules ina planar texture along the electric field lines. A typical lightmodulation element of this type is schematically illustrated in FIG. 1,wherein 11 is a layer of an LC medium, 12 and 13 are linear polarisers,14 are the electrodes and 15 depicts the electric field lines. FIG. 1Ashows the element in its driven, light state where the LC material is inits liquid crystal phase. FIG. 1B shows the element in its undriven,dark state, where the LC material is in an optically isotropic phase,preferably in a blue phase. The field-induced birefringence of the LClayer retards the incident linearly polarised light, enabling it to passthrough the second polariser. The optics of the light state can beapproximated to that of a planar aligned LC between crossed polarisers.Although the regions directly above the electrodes are homeotropic, themajority of the LC is planar aligned. The optics of the dark state isthat of an optically isotropic medium between crossed polarisers.

ISP LCDs, details on their assembly and suitable components and LC mediato be used therein are described in WO 02/93244 A1, DE 10217273 A1, DE102 41 301 and DE 103 139 79, the entire disclosure of which isincorporated into this application by reference.

ISP LCDs are suitable, inter alia, as display screens of televisionsets, computers, such as, for example, notebook computers or desktopcomputers, central control units and of other equipment, for examplegambling machines, electro-optical displays, such as displays ofwatches, pocket calculators, electronic (pocket) games, portable databanks, such as PDAs (personal digital assistants) or of mobiletelephones.

However, LCDs of the ISP mode often show a limited viewing angleperformance. For example, in a typical ISP mode display as shown in FIG.1, in specific directions the dark state often exhibits light leakagewhich results in contrast reduction. In addition, the light stateluminance is often reduced and colouration can occur. Also, the contrastis often limited which is a disadvantage especially in large areaapplications such as television.

The inventors of the present invention have found that the properties ofISP LCDs, especially the viewing angle characteristics, can be furtherimproved by using a compensator comprising one or more retardation filmsof specific types and in specific arrangements as described in thefollowing invention. In particular, this invention relates to opticalfilms which can be applied to ISP LCDs to improve the viewing angle interms of dark state luminance (and hence contrast) and surprisingly, thewhite-state luminance and colour.

One aim of the present invention is to provide a compensator for a lightmodulation element or display of the ISP mode with improved opticalperformance. The compensator should preferably be easy to manufactureand allow economic fabrication even at large scales.

Another aim of this invention is to provide an advantageous use of thecompensator according to this invention.

Another aim of this invention is to provide a light modulation elementor display of the ISP mode comprising an inventive compensator whichshows advantageous properties such as good contrast, reduced colourshift and wide viewing angles.

Other aims of the present invention are immediately evident to theperson skilled in the art from the following detailed description.

The above aims can be achieved by providing compensators, lightmodulation elements and displays according to the present invention asdescribed above and below.

DEFINITION OF TERMS

In connection with polarisation, compensation and retardation layers,films or plates as described in the present application, the followingdefinitions of terms as used throughout this application are given.

The term ‘film’ includes self-supporting, i.e. free-standing, films orlayers of material that show more or less pronounced mechanicalstability and flexibility, as well as coatings or layers on a supportingsubstrate or between two substrates.

The term ‘liquid crystal or mesogenic material’ or ‘liquid crystal ormesogenic compound’ should denote materials or compounds comprising oneor more rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e.groups with the ability to induce liquid crystal phase behaviour. Liquidcrystal (LC) compounds with rod-shaped or board-shaped groups are alsoknown in the art as ‘calamitic’ liquid crystals. Liquid crystalcompounds with a disk-shaped group are also known in the art as‘discotic’ liquid crystals. The compounds or materials comprisingmesogenic groups do not necessarily have to exhibit a liquid crystalphase themselves. It is also possible that they show liquid crystalphase behaviour only in mixtures with other compounds, or when themesogenic compounds or materials, or the mixtures thereof, arepolymerised.

For the sake of simplicity, the term ‘liquid crystal material’ is usedfor both liquid crystal materials and mesogenic materials, and the term‘mesogen’ is used for the mesogenic groups of the material.

The term ‘director’ means the preferred orientation direction of thelong molecular axes in case of calamitic compounds, or of the shortmolecular axis in case of discotic compounds, of the mesogens in aliquid crystal material.

The term ‘planar structure’ or ‘planar orientation’ refers to a layer ofoptically anisotropic material wherein the optical axis is substantiallyparallel to the plane of the layer.

The term ‘homeotropic structure’ or ‘homeotropic orientation’ refers toa layer of optically anisotropic material wherein the optical axis issubstantially perpendicular to the plane of the layer.

The terms ‘tilted structure’ or ‘tilted orientation’ refers to a layerof optically anisotropic material wherein the optical axis is tilted atan angle θ between 0 and 90 degrees relative to the plane of the layer.

The term ‘splayed structure’ or ‘splayed orientation’ means a tiltedorientation as defined above, wherein the tilt angle within the layervaries monotonuously in the range from 0 to 90°, preferably from aminimum to a maximum value, in a direction perpendicular to the plane ofthe layer.

Unless stated otherwise, the tilt angle of a splayed layer is given asthe average tilt angle θ_(ave) which is defined as$\theta_{ave} = \frac{\sum\limits_{d^{\prime} = 0}^{d}{\theta^{\prime}\left( \mathbb{d}^{\prime} \right)}}{\mathbb{d}}$wherein θ′(d′) is the local tilt angle at the thickness d′ within thelayer, and d is the total layer thickness.

In planar, homeotropic and tilted optical films or layers comprisinguniaxially positive birefringent liquid crystal material with uniformorientation, the direction of the optical axis is given by the directorof the liquid crystal material.

The term ‘uniaxial’ refers to a material, film or layer having threeprincipal refractive indices in directions orthogonal to each other,wherein two refractive indices are identical and the third refractiveindex is different from the other two refractive indices, resulting in asingle, unique optic axis.

The term ‘biaxial’ refers to a material, film or layer having threeprincipal refractive indices in directions orthogonal to each other,wherein the three refractive indices are different, resulting in twooptic axes.

The term ‘A plate’ refers to an optical retarder utilizing a layer ofbirefringent material with its optical axis oriented parallel to theplane of the layer.

The term ‘twisted A plate’ refers to an A plate comprising molecularsublayers, each having an optical axis that is defined by an azimuthalor twist angle measured relative to a reference axis in the plane of thelayer, wherein said azimuthal or twist angle varies monotonuously in adirection perpendicular to the layer.

The term ‘C plate’ refers to an optical retarder utilizing a layer ofbirefringent material with its optical axis perpendicular to the planeof the layer.

The term ‘O plate’ refers to an optical retarder utilizing a layer ofbirefringent material with its optical axis oriented at an oblique anglewith respect to the plane of the layer.

The terms ‘tilted O plate’ and ‘splayed O plate’ refer to an O platehaving tilted or splayed structure, respectively, as defined above.

In A-, C- and O-plates comprising optically uniaxial birefringent liquidcrystal material with uniform orientation, the direction of the opticalaxis is usually identical to the direction of the extraordinary axis.

An A-, C- or O-plate comprising optically uniaxial birefringent materialwith positive birefringence is also referred to as ‘+A/C/O plate’ or‘positive A/C/O plate’. An A-, C- or O-plate comprising opticallyuniaxial birefringent material with negative birefringence is alsoreferred to as ‘− A/C/O plate’ or ‘negative A/C/O plate’.

A retardation film with positive or negative birefringence is alsoreferred to as ‘positive’ or ‘negative’ retardation film, respectively.

SUMMARY OF THE INVENTION

The present invention relates to an electro-optical light modulationelement comprising an electrode arrangement, at least one element forpolarisation of the light, and a mesogenic modulation medium, said lightmodulation element being operated at a temperature at which themesogenic modulation medium in the unaddressed state is in an opticallyisotropic phase, characterized in that it comprises at least onecompensation element comprising at least one birefringent polymer film.

The invention further relates to an electro-optical light modulationelement comprising an electrode arrangement, at least one element forpolarisation of the light, and a mesogenic modulation medium, said lightmodulation element being operated at a temperature at which themesogenic modulation medium in the unaddressed state is in an opticallyisotropic phase, characterized in that it comprises at least onecompensation element comprising

-   a) at least one optical retardation layer having an optical axis    that is substantially parallel to the plane of the layer and to the    surface of the mesogenic modulation medium, and/or-   b) at least one optical retardation layer having an optical axis    that is substantially perpendicular to the plane of the layer and to    the surface of the mesogenic modulation medium, and/or-   c) at least one optical retardation layer having an optical axis    that is tilted at an angle θ between 0° and 90° relative to the    plane of the layer and to the surface of the mesogenic modulation    medium and/or-   d) at least one optical biaxial retardation layer.

The invention further relates to an electro-optical display comprisingone or more light modulation elements as described above and below.

The invention further relates to an optical compensation element for usein a light modulation element or display as described above and belowcomprising at least one birefringent polymer film or comprising at leastone layer a) and/or at least one layer b) and/or at least one layer c)and/or at least one layer d) as described above and below.

The invention further relates to the use of a light modulation elementor display as described above and below for the display of informationor of video signals, as monitor like for example television or computermonitor, or for projection systems.

DETAILED DESCRIPTION OF THE INVENTION

The light modulation element or display according to the presentinvention is operated at a temperature where the mesogenic modulationmedium is in an optically isotropic phase, like for example the bluephase or the isotropic phase.

In a preferred embodiment of the present invention the light modulationelement or display is operated in the isotropic phase of the mesogenicmodulation medium, i.e. at a temperature above the clearing temperatureof the medium.

In a preferred embodiment of the present invention the light modulationelement or display is operated in the blue phase, or one of the bluephases, of the mesogenic modulation medium.

Preferably the electrode arrangement in the light modulation element ordisplay according to the present invention is able to generate anelectric field having a significant component parallel to the surface ofthe mesogenic modulation medium.

Very preferably the electrode arrangement during operation of the lightmodulation element generates an electric field having a significantcomponent parallel to the plane of the mesogenic modulation medium.

Preferably the electrode arrangement is located on one side of the layerof the mesogenic modulation medium.

Preferably the light modulation element or display further comprises twoelements for polarisation of the light, preferably linear polarisers,sandwiching the mesogenic modulation medium, so that light duringpassage through the light modulation element in each case passes throughat least one polariser before passing through the mesogenic modulationmedium and after passing through the mesogenic modulation medium.

In another preferred embodiment of the present invention the lightmodulation element or display is a multidomain display, very preferablycomprising at least two domains wherein the preferred orientationdirections of the molecules in the mesogenic modulating medium areparallel to the plane of the medium and orthogonal to each other.Preferably said preferred orientation directions form an angle of 45° tothe polariser orientations. Such compensated and uncompensated dualdomain or multi domain elements and displays have especially improvedwhite state luminance and colour.

Preferably the light modulation element or display comprises a liquidcrystal medium. The mesogenic or liquid crystal medium preferably has anematic or smectic, very preferably a nematic phase.

The optical performance of the light modulation element or displayaccording to the present invention is improved by the use of an opticalcompensation element as described above and below, hereinafter referredto as compensator.

The use of the compensator in the light modulation element or displayespecially leads to better viewing angle characteristics, like goodcontrast and high level stability at wide viewing angles and reducedcolour shift. Furthermore, it is possible to improve the viewing angleof in terms of dark state luminance (and hence contrast) and white-stateluminance and colour.

The compensator comprises one or more retardation layers which can befor example compensation or retardation films comprising a birefringentmaterial in its solid state, like for example birefringent polymerfilms. It is also possible to use as compensation element for example alayer of a birefringent mesogenic or liquid crystal medium comprisinglow molecular weight compounds, which is preferably confined between twotransparent substrates and aligned into uniform orientation.

Especially preferred is a compensator comprising a retardation orcompensation layer or film comprising birefringent polymer material.

Further preferred is a compensator comprising a retardation orcompensation layer or film comprising polymerised or crosslinkedmesogenic or liquid crystal (LC) material. As the optical retardation ofa birefringent layer is defined as the product d·Δn of the layerthickness d and the birefringence Δn, the use of LC materials with highbirefringence allows to reduce the layer thickness, or, at constantlayer thickness, to increase the optical retardation. Furthermore, byappropriate choice of the LC material of the retardation layer itsoptical properties can be adapted to those of the LC material in thelight modulation element or display, which allows improved compensation.

In a preferred embodiment of the present invention the compensatorcomprises at least one layer a) comprising one or more retardation filmshaving an optical axis that is substantially parallel to the film plane,hereinafter also referred to as A plate.

The A plate can be a positive A plate, wherein the refractive index inthe direction parallel to the optical axis is larger than in directionsperpendicular to the optical axis, or a negative A plate, wherein therefractive index in the direction parallel to the optical axis issmaller than in directions perpendicular to the optical axis.

Especially preferably the compensator comprises at least one positive Aplate.

The optical axis of the A plate is preferably parallel to thetransmission axis of the adjacent polariser.

Suitable optical films for use as A plate are known in prior art, likefor example uniaxially stretched polymer films such aspolyethyleneterephthalate (PET), polyvinylalcohol (PVA) or polycarbonate(PC) films.

The A plate preferably comprises polymerised LC material with planarstructure as described for example in WO 98/04651, the entire disclosureof which is incorporated by reference.

In another preferred embodiment the compensator comprises at least onelayer b) comprising one or more retardation films having an optical axisthat is substantially perpendicular to the film plane, hereinafter alsoreferred to as C plate.

The C plate can be a positive C plate, wherein the refractive index inthe direction parallel to the optical axis is larger than in directionsperpendicular to the optical axis, or a negative C plate, wherein therefractive index in the direction parallel to the optical axis issmaller than in directions perpendicular to the optical axis.

Especially preferably the compensator comprises at least one positive Cplate.

As positive C plate preferably a film is used that comprises polymerisedLC material with homeotropic structure, as described for example in WO98/00475, the entire disclosure of which is incorporated into thisapplication by reference.

Suitable optical films for use as negative C plate are known in priorart, like for example stretched or uniaxially compressed plastic filmslike DAC or TAC as described for example in U.S. Pat. No. 4,701,028,inorganic thin films obtained by physical vapour deposition as describedfor example in U.S. Pat. No. 5,196,953, or negatively birefringentpolyimide films as described for example in U.S. Pat. No. 5,480,964 andU.S. Pat. No. 5,395,918.

As negative C plate preferably a film is used that comprises polymerisedchiral LC, in particular cholesteric LC (CLC) material having acholesteric helix axis substantially perpendicular to the film plane andhaving a short pitch and a reflection in the UV range, like for examplea UVCLC film or highly twisted A plate as described in GB 2,315,072 andWO 01/20394, the entire disclosure of which is incorporated into thisapplication by reference. A UVCLC film as described in these documentsdoes not have the same structure as a conventional negative C plate withan extraordinary axis oriented perpendicular to the film plane, butnevertheless has the same optical properties as a negative C plate.

In another preferred embodiment the compensator comprises at least onelayer c) comprising one or more retardation films having an optical axisthat is tilted at an angle θ between 0° and 90° relative to the filmplane, hereinafter also referred to as O plate. Especially preferred arepositive O plates. The O plate can have a tilt angle that issubstantially constant throughout the film, or a tilt angle that variesmonotonuously in a direction normal to the film plane, the latter ofwhich is also known as splayed O plate.

Especially preferred are splayed O plates.

In another preferred embodiment the compensator comprises two splayed Oplate layers with the average tilt of both layers being in the sameplane and having opposing direction. This embodiment is illustrated inFIGS. 2A and 2B and relates to two splayed layers 21 and 22 wherein theoptical axes 23 and 24 in different parts of both layers are in the sameplane and the tilt angle θ, when looking at the layers in side view andwhen going from low values to high values of θ, has opposite sense ofvariation, i.e. clockwise and counterclockwise respectively, in the twolayers.

Especially preferred are the specific arrangements shown in FIGS. 2A and2B.

The arrow in FIGS. 2A and 2B indicates the preferred direction of lightfrom the backlight to the viewer.

In case the two splayed layers in the embodiment as illustrated in FIGS.2A and 2B have the same average tilt angle and the same minimum andmaximum tilt, the directions 25 and 26 of average tilt in the two layersshow mirror symmetry.

Preferably the projection of the optical axis of the O plate to theplane of the film is parallel to the direction of either of thepolariser transmission directions.

Preferably the two splayed O plates have the same average tilt angle,very preferably the same average tilt angle and the same minimum andmaximum tilt angle.

In another preferred embodiment the compensator comprises two negative Oplates which can be tilted or splayed.

Suitable films for use as O plate retarders are known in prior art, andcan be obtained for example by oblique vapour deposition of a thin film,e.g. of an inorganic material such as Ta₂O₅, as described for example inU.S. Pat. No. 5,196,953 and WO 96/10773. It is also possible to use as Oplate an LC film as described in WO 96/10770, which is prepared from apolymerisable LC material with a smectic A or C phase and a nematicphase at higher temperatures, by applying the LC material in its nematicphase onto a substrate optionally covered with an alignment layer ofobliquely deposited SiO, lowering the temperature into the smectic Cphase of the material so that the LC material adopts its naturallytilted smectic C structure, and fixing the tilted structure bypolymerisation of the LC material.

The tilted or splayed O plate preferably comprises a polymerised LCmaterial with tilted or splayed structure, as described for example inU.S. Pat. No. 5,619,352, WO 97/44409, WO 97/44702, WO 97/44703 or WO98/12584, the entire disclosure of which is incorporated by reference.

In another preferred embodiment the compensator comprises at least onelayer d) comprising one or more retardation films having three differentrefractive indices in directions perpendicular to each other,hereinafter also referred to as biaxial film.

Preferably the biaxial film has n_(x)≠n_(y)≠n_(z) and n_(y)>n_(z)>n_(x)wherein n_(x) and n_(y) are the principal refractive indices inorthogonal directions in the film plane and n_(z) is the principalrefractive index perpendicular to the film plane.

Preferably the direction of n_(x) or n_(y) is parallel to either of thepolarisers transmission direction.

Further suitable biaxial films are for example biaxially stretched orside-stretched polymer films.

The single retardation films in a compensator, light modulation elementor display according to the present invention can be laminated directlyonto each other or onto the other optical components, or separated bytransparent intermediate films, like for example TAC, DAC or PVA filmsor by adhesive layers like for example pressure sensitive adhesives(PSA) which are commercially available.

The polarising elements are preferably linear polarisers, verypreferably standard dichroic polarisers. Such films are known in priorart and are commercially available. It is also possible to use linearpolarisers comprising polymerised liquid crystal material with planarorientation.

Especially preferred is a compensator, light modulation element ordisplay comprising

-   -   at least one, preferably one positive A plate,    -   at least one, preferably one negative A plate,    -   at least one, preferably one positive C plate,    -   at least one, preferably one positive A plate and at least one,        preferably one positive C plate, which can be located on the sam        side or on opposite sides of the modulating medium,    -   at least one, preferably one O plate,    -   at least two, preferably two O plates, very preferably two        splayed O plates, most preferably two splayed O plates oriented        anti-parallel to each other, preferably located on opposite        sides of the modulating medium,    -   at least one, preferably one biaxial film,    -   an element or display comprising two or more, preferably two        biaxial films, preferably located on opposite sides of the        modulating medium,    -   an A plate and/or C plate and/or 0 plate and/or biaxial film        comprising polymerised or crosslinked LC material.

Table 1 shows especially preferred compensator stacks in a lightmodulation element or display according to the present invention.Therein, P denotes a polarising element, LC denotes a switchable LC cellof the ISP mode comprising the electrodes and the mesogenic or liquidcrystal medium, A denotes an A plate, C denotes a C plate, Bi denotes abiaxial film, TAC denotes a TAC film. “+” and “−” denote positive andnegative retarders. Brackets indicate an element that can also beomitted. Unless stated otherwise, the direction of incident light isfrom left to right. TABLE 1 1) P (TAC) +A LC (TAC) P 2) P (TAC) LC +A(TAC) P 3) P (TAC) +C LC (TAC) P 4) P (TAC) LC +C (TAC) P 5) P (TAC) −ALC (TAC) P 6) P (TAC) LC −A (TAC) P 7) P (TAC) +C LC +A (TAC) P 8) P(TAC) +A LC +C (TAC) P 9) P (TAC) +A +C LC (TAC) P 10) P (TAC) LC +C +A(TAC) P 11) P (TAC) Bi LC (TAC) P 12) P (TAC) LC Bi (TAC) P 13) P (TAC)Bi LC Bi (TAC) P 14) P (TAC) O → LC

O (TAC) P

The compensator, light modulation element or display according to thepresent invention may further comprise one or more further opticalcomponents such as polarisers or compensation or retardation films, likefor example one or more quarter wave retardation films (QWF, λ/4 films)or half wave retardation films (HWF, λ/2 films), positive or negative A,O or C plates or retardation films with twisted, homeotropic, planar,tilted or splayed structure. Particularly preferred are optical filmscomprising polymerised or crosslinked LC material.

The light modulation element or display according to the presentinvention may be a reflective or transmissive device, and may furthercomprise a light source, like a conventional backlight, or a reflectivelayer on the side of the modulating medium opposite to that of the firstlinear polariser. In case of a reflective display with a reflectivelayer on one side of the modulating medium the second linear polarisermay be omitted.

The retardation films like the A plate, C plate, 0 plate and biaxialretarders in the compensator, light modulation element or displayaccording to the present invention preferably comprise polymerised orcrosslinked LC material. The polymerised LC material can comprisecalamitic or discotic liquid crystal compounds. Suitable calamiticmaterials are described for example in WO 98/04651, WO 98/00475, WO01/20394, WO 98/12584. Suitable discotic materials are described forexample in U.S. Pat. No. 5,730,900 and U.S. Pat. No. 5,635,105.

The polymerised LC material can comprise LC moieties or LC compounds aspart of the polymer main chain or as part of the polymer side chain.

The retardation films are preferably prepared from a polymerisable LCmaterial by in-situ polymerisation. In a preferred method of preparationthe polymerisable LC material is coated onto a substrate, oriented intothe desired orientation and subsequently polymerised for example byexposure to heat or actinic radiation as described for example in WO01/20394, GB 2,315,072 or WO 98/04651.

Alternatively it is possible to prepare the retardation films from areadily synthesized LC polymer that is applied onto a substrate, forexample at a temperature above its glass transition temperature or itsmelting point, or from solution e.g. in an organic solvent, aligned intothe desired orientation, and solidified for example by evaporating thesolvent or by cooling below the glass temperature or melting point ofthe LC polymer. If for example a LC polymer with a glass temperaturethat is higher than ambient temperature is used, evaporation of thesolvent or cooling leaves a solid LC polymer film. If for example an LCpolymer with a high melting point is used, the LC polymer can be appliedas a melt onto the substrate which solidifies upon cooling. LC sidechain polymers or LC main chain polymers can be used, preferably LC sidechain polymers. The LC polymer should preferably be selected such thatits glass transition or melting temperature is significantly higher thanthe operating tempature of the retarder. For example, LC side chainpolymers comprising a polyacrylate, polymethacrylate, polysiloxane,polystyrene or epoxide backbone with laterally attached mesogenic sidechains can be used. The LC polymer may also comprise side chains withreactive groups that can be crosslinked after or during evaporation ofthe solvent to permanently fix the orientation. The LC polymer may alsobe subjected to mechanical or heat treatment after application to thesubstrate to improve alignment. Suitable LC polymers for this method areknown to the ordinary expert.

Further suitable methods and materials for the preparation ofretardation films are known to those skilled in the art.

The examples below serve to illustrate the invention without limitingit. Therein, the following abbreviations are used:

-   n_(x), n_(y) principal refractive indices in orthogonal directions    in the film plane-   n_(z) principal refractive index perpendicular to the film plane-   Δn birefringence-   d layer thickness [μm]-   d×Δn optical retardation [nm]

Unless stated otherwise, values of n are given for 20° C. and 550 nm.

The uncompensated and compensated displays described in the followingexamples, unless explicitly stated otherwise, contain the followingcomponents: LC cell ISP mode LC cell with the following cell parametersCell gap d = 6400 nm Δn (550 nm) = 0.036 (light state), 0.000 (darkstate) Light State = Planar Aligned LC at 45° Dark State = OpticallyIsotropic Polarisers = Aligned at 0° and 90° TAC (n_(x) − n_(y)) · d =2.3 nm TAC (n_(y) − n_(z)) · d = 53 nm Phi = Angle from normal to thecell plane Theta = Azimuthal angle within the cell planePolarisers

Standard type stretched dichroic polarisers, the stretch axis beingindicated in the Figures by double headed arrows.

All the following examples include TAC layers in the polarisers, whichact as slightly biaxial −C retarders. These can be removed from thestack and by appropriately combining the TAC retardation with thecompensators, a good viewing angle can be maintained.

The display configurations of the following examples also contain alight source, like for example a backlight on the left side of thedisplay (not shown in the Figure).

The values and plots of the iso-contrast and grey levels in thefollowing examples are obtained by modelling or measurement,respectively, using berreman matrix methods for optical simulations andEldim EZContrast equipment for viewing angle measurement.

EXAMPLE 1 (COMPARATIVE EXAMPLE) Uncompensated Display

An uncompensated display of the ISP mode as shown in FIG. 1 comprises anLC cell and two polarisers as defined above.

The light state luminance, dark state luminance, isocontrast plot andlight state colour shift are shown in FIGS. 3A, 3B, 3C and 3D,respectively. The orientation of the LC director in the LO medium isdepicted by the arrow in FIG. 3A.

The dark state exhibits light leakage in the 45° directions, whichresults in contrast reduction. In addition, the light state luminance isreduced and colouration occurs in the direction perpendicular to the LCdirector. The 10:1 contrast region only extends to 60° in the 45°directions, making this uncompensated mode unsuitable for large areaapplications such as television.

EXAMPLE 2 +A Plate Compensation

A compensated display according to stack (1) in table 1 wherein both TACfilms are present.

+A-plate: d·Δn=340 nm

+A-plate orientation: theta=90°, phi=90°

The light state luminance, dark state luminance, isocontrast plot andlight state colour shift are shown in FIGS. 4A, 4B, 4C and 4D,respectively.

EXAMPLE 3 +C Plate Compensation

A compensated display according to stack (4) in table 1 wherein both TACfilms are present.

+C-plate: d·Δn=124 nm

+C-plate orientation: theta=0

The light state luminance, dark state luminance, isocontrast plot andlight state colour shift are shown in FIGS. 5A, 5B, 5C and 5D,respectively.

EXAMPLE 4 −A Plate Compensation

A compensated display according to stack (6) in table 1 wherein both TACfilms are present.

−A-plate: d·Δn=194 nm

−A-plate orientation: theta=90°, phi=90°

The light state luminance, dark state luminance, isocontrast plot andlight state colour shift are shown in FIGS. 6A, 6B, 6C and 6D,respectively.

EXAMPLE 5 +A Plate & +C Plate Compensation

A compensated display according to stack (7) in table 1 wherein both TACfilms are present.

-   +A-plate: d·Δn=85 nm-   +A-plate orientation: theta=90°, phi=90°-   +C-plate: d·Δn=179 nm-   +C-plate orientation: theta=0°

The light state luminance, dark state luminance, isocontrast plot andlight state colour shift are shown in FIGS. 7A, 7B, 7C and 7D,respectively.

EXAMPLE 6 Biaxial Compensation

A compensated display according to stack (12) in table 1 wherein bothTAC films are present.

-   Biaxial Film: d·(n_(y)−n_(x))=269 nm d·(n_(z)−n_(y))=−121 nm-   Orientation: theta=0°, phi=0°

The light state luminance, dark state luminance, isocontrast plot andlight state colour shift are shown in FIGS. 8A, 8B, 8C and 8D,respectively.

EXAMPLE 7 Dual Biaxial Compensation

A compensated display according to stack (13) in table 1 wherein bothTAC films are present. Biaxial film 1 is on the left side, biaxial film2 on the right side of the LC cell.

-   Biaxial Film 1: theta=0°, phi=0°    -   d·(n_(y)−n_(x))=116 nm    -   d·(n_(z)−n_(y))=−146 nm-   Biaxial Film 2: theta=90°, phi=0°    -   d·(n_(y)−n_(x))=−59 nm    -   d·(n_(z)−n_(y))=40 nm

The light state luminance, dark state luminance, isocontrast plot andlight state colour shift are shown in FIGS. 9A, 9B, 9C and 9D,respectively.

Example 8 Dual Splayed +O-Plate Compensation

A compensated display according to stack (14) in table 1 wherein bothTAC films are present. The splay configuration of the two O-plates is asshown in FIG. 2A.

-   Splayed +O-plate 1: Lower surface: theta=86°, phi=0°    -   Upper surface: theta=15°, phi=0°    -   On axis: d·Δn=119 nm-   Splayed +O-plate 2: Lower surface: theta=15°, phi=0°    -   Upper surface: theta=86°, phi=0°    -   On axis: d·Δn=119 nm

The light state luminance, dark state luminance, isocontrast plot andlight state colour shift are shown in FIGS. 10A, 10B, 10C and 10D,respectively.

Example 9 (Comparative Example) Uncompensated Dual Perpendicular Domains

An uncompensated ISP mode display with two domains wherein the preferredorientation direction of the LC molecules within the cell plane is is 0°and 90°, respectively, when an electric field is applied.

The light state luminance, dark state luminance, isocontrast plot andlight state colour shift are shown in FIGS. 11A, 11B, 11C and 11D,respectively.

Example 10 Dual Perpendicular Domains with +A-plate & +C-PlateCompensation

A compensated ISP mode display with two domains wherein the preferredorientation direction of the LC molecules within the cell plane is is 0°and 90°, respectively, when an electric field is applied, according tostack (7) in table 1 wherein both TAC films are present.

-   +A-plate: d·Δn=85 nm-   +A-plate orientation: theta=90°, phi=90°-   +C-plate: d·Δn=179 nm-   +C-plate orientation: theta=0°

The light state luminance, dark state luminance, isocontrast plot andlight state colour shift are shown in FIGS. 12A, 12B, 12C and 12D,respectively.

Examples 2-8 show that an excellent dark state over all viewing anglescan be achieved by the use of appropriate compensation films. Thisresults in a theoretical greater than 100:1 contrast in all viewingdirections. Application of compensation films also has the surprisingeffect that the off-axis luminance in the white state is improved aswell as the off-axis chromaticity.

Examples 8 and 9 show the effect of a two domain white state. The twodomains have perpendicular LC directors that lie 45° and 135° relativeto the polarisers. This preferred cell configuration, especially whencompensated, achieves superior light-state colour and improved luminanceat wide viewing angles.

1. Electro-optical light modulation element comprising an electrodearrangement, at least one element for polarisation of the light, and amesogenic modulation medium, said light modulation element beingoperated at a temperature at which the mesogenic modulation medium inthe unaddressed state is in an optically isotropic phase, characterizedin that it comprises at least one optical compensation elementcomprising at least one birefringent polymer film.
 2. Electro-opticallight modulation element comprising an electrode arrangement, at leastone element for polarisation of the light, and a mesogenic modulationmedium, said light modulation element being operated at a temperature atwhich the mesogenic modulation medium in the unaddressed state is in anoptically isotropic phase, characterized in that it comprises at leastone optical compensation element comprising a) at least one opticalretardation layer having an optical axis that is substantially parallelto the plane of the layer and to the surface of the mesogenic modulationmedium, and/or b) at least one retardation layer having an optical axisthat is substantially perpendicular to the plane of the layer and to thesurface of the mesogenic modulation medium, and/or c) at least oneretardation layer having an optical axis that is tilted at an angle □ ofbetween 0° and 90° relative to the plane of the layer and to the surfaceof the mesogenic modulation medium, and/or d) at least one opticalbiaxial retardation layer.
 3. Electro-optical light modulation elementaccording to claim 1, characterized in that the electrode arrangement islocated on one side of the layer of the mesogenic modulation medium andduring operation of the light modulation element generates an electricfield having a significant component parallel to the plane of themesogenic modulation medium.
 4. Electro-optical light modulation elementaccording to claim 1, characterized in that the light during passagethrough the light modulation element in each case passes through atleast one polariser before passing through the mesogenic modulationmedium and after passing through the mesogenic modulation medium. 5.Electro-optical light modulation element according to claim 1,characterized in that it comprises at least two domains wherein thepreferred orientation directions of the molecules in the mesogenicmodulating medium are parallel to the plane of the medium and orthogonalto each other.
 6. Electro-optical light modulation element according toclaim 1, characterized in that the mesogenic modulation medium has anematic liquid crystal phase.
 7. Electro-optical light modulationelement according to claim 2, characterized in that in said compensationelement at least one of the optical retardation layers a) and/or b)and/or c) and/or d) is a birefringent polymer film.
 8. Electro-opticallight modulation element according to claim 1, characterized in that insaid compensation element the birefringent polymer film or at least oneof the optical retardation layers a) and/or b) and/or c) and/or d)comprises polymerised or crosslinked LC material.
 9. Electro-opticallight modulation element according to claim 2, characterized in thatsaid compensation element comprises at least one layer a) comprising atleast one positive A plate retardation film.
 10. Electro-optical lightmodulation element according to claim 2, characterized in that saidcompensation element comprises at least one layer a) comprising at leastone negative A plate retardation film.
 11. Electro-optical lightmodulation element according to claim 2, characterized in that saidcompensation element comprises at least one layer b) comprising at leastone positive C plate retardation film.
 12. Electro-optical lightmodulation element according to claim 2, characterized in that saidcompensation element comprises at least one layer c) comprising at leastone positive O plate retardation film.
 13. Electro-optical lightmodulation element according to claim 2, characterized in that saidcompensation element comprises at least one layer d) comprising at leastone biaxial retardation film.
 14. Electro-optical light modulationelement according to claim 2, characterized in that said compensationelement comprises at least layer a) comprising at least one positive Aplate retardation film and at least one layer b) comprising at least onepositive C plate retardation film.
 15. Electro-optical light modulationelement according to claim 2, characterized in that said compensationelement comprises at least two layers c) comprising at least onepositive splayed O plate retardation film with the average tilt of bothlayers being in the same plane and having opposing direction. 16.Electro-optical light modulation element according to claim 2,characterized in that said compensation element comprises at least twolayers d) comprising at least one biaxial retardation film. 17.Electro-optical display comprising one or more light modulation elementsaccording to claim
 1. 18. Use of a light modulation element or displayaccording to claim 1 for the display of information or of video signals,as monitor, television or computer monitor, or for projection systems.19. Compensation element of this invention for use in a light modulationelement or display according to claim 1.